1. Field of the Invention
The present invention relates to absolute porosity and pore size determination of pore types with varying pore sizes in porous media from subsurface formations, and more particularly to absolute porosity and pore size determination with a Nuclear Magnetic Relaxation Dispersion (NMRD) profile from a Fast Field Cycling Nuclear Magnetic Resonance (FFC-NMR) measure.
2. Description of the Related Art
Accurate knowledge of formation rock porosity is important. The value of estimating accurate reserves by identifying accurate porosities with movable fluids is very important. In addition, the accurate measurement of actual pore size determination with non-destructive method adds great value for various petrophysical properties measurements.
Accurate measurement of porosity and pore size corresponding to each pore type is important for reservoir characterization. Porous media with various pore sizes are common, such as carbonate rocks encountered in hydrocarbon producing formations of reservoirs. Non-destructive methods for measuring the absolute porosity of each pore type, however, are not available, as far as is known. Since these pores with different sizes are interconnected to each other by diffusion coupling, the measurement of true porosity of each pore type is not a trivial process.
Due to the lack of absolute porosity data for different pore types in reservoir rocks, mostly carbonates, the current reserve estimations methods based on porosity data have suffered from a considerable margin of errors.
Pore throat size distribution can be provided by laboratory mercury injection capillary pressure (MICP) data, but pore throat size is different from pore body size. Porosity measurement by micro-CT (microcomputed tomography) can provide the pore size and absolute porosity of each pore type, but it requires a contrast cutoff value to differentiate between pore and matrix. However, micro-CT is not a physical measurement, but a data estimation process. In addition the sample size of MICP and micro-CT are usually too small to be a representative volume of a formation rock with inhomogeneous pore size distribution.
Pore throat size distribution determination with low-field NMR relaxometry methods has also been attempted. However, the results have been subject to a certain degree of uncertainly due to the data analysis by 1D and 2D inversion processes.
Prior art techniques have been able to distinguish between fluid types (hydrocarbon and water) in rock samples pores by identifying biphasic behavior of water and hydrocarbon, but so far as is known not to identify the absolute porosity and pore size determination of each pore type.